Add better support for State types that span multiple media

[bctcvai]

Often state elements that represent a duration of time occur in the system, it is better to link those across multiple media vs. have multiple states be clones of each other in cases when the states are mutable.

Evaluate whether changes to the state model have to occur to better represent time-based events.

[jrtcppv]

Proposed approach:

• Add a new interpolation value attr_style_time_range (analog to attr_style_range)
• Add new datetime dtype styles start_time and end_time
• The UI will use either the media filename or a media attribute with style start_time to determine the start and end frame within the media.